The separation of various substances through selective adsorption is an important process for producing pure substances. The development of simulated moving bed (SMB) technology, the adsorption separation process can be operated on a continuous basis and for large volumes of materials to be separated and is used in a variety of processes in the petrochemical industry. For simulated moving bed technology, the process uses a multiport rotary valve to redirect flow lines in the process. The simulation of a moving adsorbent bed is described in U.S. Pat. No. 2,985,589 (Broughton et al.). In accomplishing this simulation, it is necessary to connect a feed stream to a series of beds in sequence, first to bed no. 1, then to bed no. 2, and so forth for numerous beds, the number of beds often being between 12 and 24. These beds may be considered to be portions of a single large bed whose movement is simulated. Each time the feed stream destination is changed, it is also necessary to change the destinations (or origins) of at least three other streams, which may be streams entering the beds, such as the feed stream, or leaving the beds. The moving bed simulation may be simply described as dividing the bed into series of fixed beds and moving the points of introducing and withdrawing liquid streams past the series of fixed beds instead of moving the beds past the introduction and withdrawal points. A rotary valve used in the Broughton process may be described as accomplishing the simultaneous interconnection of two separate groups of conduits. There are numerous patents and publications describing the mechanical aspects and internals of the SMB apparatus.
The use of simulated moving beds is important for the separation of xylenes, and especially para-xylene from a mixture of xylenes and other C8 aromatic compounds, such as ethylbenzene. U.S. Pat. No. 3,686,342 issued to R. W. Neuzil describes the separation of para-xylene from a mixture of xylenes using a zeolitic adsorbent and with para-diethylbenzene as the desorbent. This is a good representation of a commercial operation. The SMB process generates a raffinate stream and an extract stream, and the handling of these streams are depicted in U.S. Pat. No. 3,510,423 issued to R. W. Neuzil et al.
Another aspect of the process is the importance of water in the separation performance. U.S. Pat. No. 5,948,950 issued to G. Hotier et al. describes the process and the importance of zeolite hydration to the separation performance. Hydration is maintained by the injection of water into one of the process streams circulating through the adsorbent. The desorbent to feed ration (S/F) disclosed in this reference varies from 0.6 to 2.5. The reference describes the use of several molecular sieve based adsorbents including barium and potassium exchanged X and Y zeolites, but does not suggest the use of a mixture of zeolites for the process. The performance of the process is measured in terms of a performance index designated IP. This reference, like the others cited above, does not discuss the importance of desorbent purity to the process performance.
There are many different process requirements in moving bed simulation processes, resulting in different flow schemes and thus variations in rotary valve arrangement. One process that is important is the production of para-xylene by separation of para-xylene from a hydrocarbon mixture comprising C8 compounds, especially from other xylenes and from ethylbenzene.
There is substantial room for improvement in the SMB process that can improve recovery of para-xylene.